1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a transflective thin film transistor substrate that reduces parasitic capacitance, and a fabricating method thereof.
2. Discussion of the Related Art
A liquid crystal display device controls a light transmittance of a liquid crystal that has dielectric anisotropy, by use of an electric field, thereby displaying a picture. The liquid crystal display device includes a liquid crystal display panel for displaying a picture through a liquid crystal cell matrix and a drive circuit for driving the liquid crystal display panel.
Referring to FIG. 1, a liquid crystal display panel of the related art includes a color filter substrate 10 and a thin film transistor substrate 20 that are bonded together with a liquid crystal 24 therebetween.
The color filter substrate 10 includes a black matrix 4, a color filter 6, and a common electrode 8 that are sequentially formed on an upper glass substrate 2. The black matrix 4 is formed in a matrix shape on the upper glass substrate 2. The black matrix 4 divides an area of the upper glass substrate 2 into a plurality of cell areas where the color filters are to be formed, and the black matrix 4 prevents light interference between adjacent cells and an external light reflection. The color filter 6 is divided into red R, green G, and blue B in the cell areas divided by the black matrix 4. The common electrode 8 formed of a transparent conductive layer on the entire surface of the color filter 6 supplies a common voltage Vcom or a reference voltage when driving a liquid crystal 24. Also, an overcoat layer (not shown) might be further formed between the color filter 6 and the common electrode 8 to level the color filter 6.
The thin film transistor substrate 20 includes a thin film transistor 18 and a pixel electrode 22 that are formed at each cell area defined by the crossing of a gate line 14 and a data line 16 on a lower glass substrate 12. The thin film transistor 18 supplies a data signal from the data line 16 to the pixel electrode 22 in response to a gate signal of the gate line 14. The pixel electrode 22 formed of the transparent conductive layer supplies the data signal from the thin film transistor 18 to the liquid crystal 24.
The liquid crystal 24 having dielectric anisotropy controls the light transmittance by rotating the liquid crystals in accordance with an electric field formed by the data signal on the pixel electrode 22 and the common voltage Vcom of the common electrode 8.
The liquid crystal display panel further includes a spacer (not shown) for uniformly maintaining a cell gap between the color filter substrate 10 and the thin film transistor substrate 20.
The color filter substrate 10 and the thin film transistor substrate 20 of the liquid crystal display panel are formed by use of a plurality of mask processes. One mask process includes many processes such as depositing (coating) a thin film, cleaning, photolithography, etching, photo-resist stripping, inspecting, etc. Specifically, the thin film transistor substrate is manufactured using a semiconductor process and requires a plurality of mask processes, thus its fabricating process is complicated is a major contributor to the manufacturing cost of the liquid crystal display panel.
Further, the liquid crystal display panels are divided into three different types: a transmission type that displays a picture by use of a light incident from a backlight unit; a reflection type that displays a picture by reflecting an external light such as a natural light; and a transflective type that combines the transmission type and the reflection type.
There is a problem in that the transmission type display consumes too much power due to the backlight and the reflection type display cannot display a picture in a dark environment because the reflection type depends on the external light. But, the transflective type display operates in a reflection mode if the external light is sufficient and in a transmission mode using the backlight unit if the external light is not sufficient, thus the power consumption can be reduced versus the transmission type display and the transflective type display is not dependent on external light like the reflection type.
To this end, the transflective type liquid crystal display panel has each pixel divided into a reflection area and a transmission area. Accordingly, the transflective thin film transistor substrate should further include a reflection electrode formed in the reflection area, and an organic insulating film formed to be relatively thick under the reflection electrode in order to equalize a light path length of the reflection area to that of the transmission area. As a result, the number of mask processes increases, so that the related art transflective thin film transistor: substrate has a problem in that its fabricating process is more complicated.
Further, the related art transflective thin film transistor substrate has a pixel electrode that overlaps both side parts of the data line, thus a parasitic capacitance is increased generating problems such as vertical cross talk, increased of power consumption etc.
FIG. 2 is a plan view illustrating a part of a transflective thin film transistor substrate according to the related art, and FIGS. 3 and 4 are cross sectional views illustrating the transflective thin film transistor substrate shown in FIG. 2, taken along the lines I-I′, II-II′.
The transflective thin film transistor shown in FIGS. 2 to 4 includes: a gate line 102 and a data line 104 that are formed on a lower substrate 142 to cross each other with a gate insulating film therebetween to define a pixel area; a thin film transistor 106 connected to the gate line 102 and the data line 104; a reflection electrode 152 formed in a reflection area of each pixel; a pixel electrode 118 formed at each pixel area connected to the thin film transistor 106 through the reflection electrode 152; a storage upper electrode 122 connected to the pixel electrode 118 through the reflection electrode 152; and a storage capacitor 120 formed to overlap the pre-stage gate line 102.
The thin film transistor 106 includes a gate electrode 108 connected to the gate line 102; a source electrode 110 connected to the data line 104; a drain electrode 112 that faces the source electrode 110 and is connected to the pixel electrode 118; an active layer 114 that overlaps the gate electrode 108 with a gate insulating film 144 therebetween to form a channel between the source electrode 110 and the drain electrode 112; and an ohmic contact layer 116 formed on the active layer 114 except in the channel region for being in ohmic contact with the source electrode 110 and the drain electrode 112.
Herein, the gate line 102 and the gate electrode 108 has a double layer structure where a first conductive layer 101 of a transparent conductive layer is deposited and a second conductive layer 103 of a metal layer is deposited thereon.
Then, a semiconductor pattern 115 including the active layer 114 and the ohmic contact layer 116 is formed to overlap the data line 104.
The reflection electrode 152 formed in the reflection area has an embossed shape in accordance with a shape of an organic insulating film thereunder, thereby increasing the reflection efficiency by a scattering effect.
The pixel electrode 118 is connected to the drain electrode 112 through the reflection electrode 152 that is formed at each pixel area and passes through an edge area of a transmission hole 154. The pixel electrode 118 has a double layer structure where the first and second conductive layers 101, 103 are deposited like the gate line 102, and the second conductive layer 103 is opened in the transmission area leaving just the first conductive layer 101 of the transparent conductive layer exposed to a transmission area.
The transmission hole 154 is formed to penetrate from an organic insulating film to the gate insulating film in the transmission area. Accordingly, the length of the light path passing through a liquid crystal layer in the reflection area becomes equal to that in the transmission area, thus the transmission efficiency of the reflection mode becomes the same as that of the transmission mode.
The storage capacitor 120 is formed by connecting a storage upper electrode 122 to the pixel electrode 118 to overlap the pre-stage gate line 102 with the gate insulating film 144 therebetween. The storage upper electrode 122 is connected to the pixel electrode 118 through the reflection electrode 152 passing through an edge area of the transmission hole 154, and the semiconductor pattern 115 further overlaps with and under the storage upper electrode 122.
In this way, in the transflective thin film transistor substrate shown in FIGS. 2 to 4, the pixel electrode 118 with the double structure is formed together with the gate line 102, and the second conductive layer 103 is etched through the reflection electrode 152, and the first conductive layer 101 is exposed in the transmission area. Further, the pixel electrode 118 is connected to the drain electrode 112 and the storage upper electrode 122 through the reflection electrode 152. Accordingly, the transflective thin film transistor substrate may be formed by four mask processes.
On the other hand, as shown in FIG. 4, the reflection electrode 156 connected to the pixel electrode 118 overlaps both sides of the data line 104 with the organic insulating film 148 and a passivation film 146 in between, thus the parasitic capacitance Cdp1, Cdp2 is increased. The vertical cross talk and the electric power consumption are increased due to the increase of the parasitic capacitance Cdp1, Cdp2.
In order to solve this problem, the transflective thin film transistor substrate according to the present invention floats the reflection electrode that overlaps the data line, thereby decreasing the parasitic capacitance.